Etching Of Solar Cell Materials

ABSTRACT

A solar cell is fabricated by etching one or more of its layers without substantially etching another layer of the solar cell. In one embodiment, a copper layer in the solar cell is etched without substantially etching a topmost metallic layer comprising tin. For example, an etchant comprising sulfuric acid and hydrogen peroxide may be employed to etch the copper layer selective to the tin layer. A particular example of the aforementioned etchant is a Co-Bra Etch® etchant modified to comprise about 1% by volume of sulfuric acid, about 4% by volume of phosphoric acid, and about 2% by volume of stabilized hydrogen peroxide. In one embodiment, an aluminum layer in the solar cell is etched without substantially etching the tin layer. For example, an etchant comprising potassium hydroxide may be employed to etch the aluminum layer without substantially etching the tin layer.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. application Ser. No.10/632,747, filed Aug. 1, 2003, which is incorporated herein byreference in its entirety.

BACKGROUND OF THE INVENTION

1. Field Of The Invention

The present invention relates generally to solar cells, and moreparticularly but not exclusively to solar cell fabrication processes andstructures.

2. Description Of The Background Art

Solar cells are well known devices for converting solar radiation toelectrical energy. They may be fabricated on a semiconductor wafer usingsemiconductor processing technology. Generally speaking, a solar cellmay be fabricated by forming p-doped and n-doped regions in a siliconsubstrate. Solar radiation impinging on the solar cell creates electronsand holes that migrate to the p-doped and n-doped regions, therebycreating voltage differentials between the doped regions. The side ofthe solar cell where connections to an external electrical circuit aremade includes a topmost metallic surface that is electrically coupled tothe doped regions. There may be several layers of materials between themetallic surface and the doped regions. These materials may be patternedand etched to form internal structures. It is important to etch thesematerials in a way that would not compromise the operability andperformance of the solar cell.

SUMMARY

A solar cell is fabricated by etching one or more of its layers withoutsubstantially etching another layer of the solar cell. In oneembodiment, a copper layer in the solar cell is etched withoutsubstantially etching a topmost metallic layer comprising tin. Forexample, an etchant comprising sulfuric acid and hydrogen peroxide maybe employed to etch the copper layer selective to the tin layer. Aparticular example of the aforementioned etchant is a Co-Bra Etch®etchant modified to comprise about 1% by volume of sulfuric acid, about4% by volume of phosphoric acid, and about 2% by volume of stabilizedhydrogen peroxide. In one embodiment, an aluminum layer in the solarcell is etched without substantially etching the tin layer. For example,an etchant comprising potassium hydroxide may be employed to etch thealuminum layer without substantially etching the tin layer.

These and other features of the present invention will be readilyapparent to persons of ordinary skill in the art upon reading theentirety of this disclosure, which includes the accompanying drawingsand claims.

DESCRIPTION OF THE DRAWINGS

FIGS. 1-4 show sectional views schematically illustrating thefabrication of a solar cell in accordance with an embodiment of thepresent invention.

FIG. 5 shows a flow diagram of a method of etching one or more materialsin a solar cell in accordance with an embodiment of the presentinvention.

The use of the same reference label in different drawings indicates thesame or like components.

DETAILED DESCRIPTION

In the present disclosure, numerous specific details are provided suchas examples of process parameters, materials, process steps, andstructures to provide a thorough understanding of embodiments of theinvention. Persons of ordinary skill in the art will recognize, however,that the invention can be practiced without one or more of the specificdetails. In other instances, well-known details are not shown ordescribed to avoid obscuring aspects of the invention.

FIGS. 1-4 show sectional views schematically illustrating thefabrication of a solar cell in accordance with an embodiment of thepresent invention. FIGS. 1-4, which are not drawn to scale, show thesolar cell in the middle of the fabrication process. Masking steps arenot shown or described in the interest of clarity.

In FIG. 1, the solar cell is shown as having a tin (Sn) layer 112, acopper (Cu) layer 110, a copper layer 108, a titanium-tungsten (TiW)layer 106, an aluminum (Al) layer 104, a silicon dioxide (SiO₂) layer102, and a silicon (Si) substrate 100. Tin layer 112 is on the backsideof the solar cell, while silicon substrate 100 is towards the front orsun side. The solar cell being fabricated is a so-called“backside-contact solar cell” in that all electrical connections to thedoped regions (not shown) in silicon substrate 100 are made from thebackside of the solar cell by way of tin layer 112. However, it is to benoted that the present invention is not limited to backside-contactsolar cells. The teachings of the present disclosure may be employed inthe fabrication of solar cells in general.

In the example of FIG. 1, tin layer 112 is the topmost metallic layerand provides a solderable metallic surface on which electricalconnections may be made. For example, interconnect leads coupled to anexternal electrical circuit or other solar cells may be soldered on tinlayer 112. Tin layer 112 also protects underlying layers of materials.For example, tin layer 112 helps prevent copper layer 110 fromcorroding. In one embodiment, tin layer 112 is electroplated to athickness of about 5 microns on copper layer 110.

Copper layer 110, copper layer 108, titanium-tungsten layer 106, andaluminum layer 104 form a Cu/TiW/Al metal stack that provides electricalconnectivity to doped regions in silicon substrate 100. In oneembodiment, copper layer 110 is electroplated to a thickness of about 20microns on copper layer 108. Masks (not shown) may be formed betweenindividual structures of copper layer 110 in gaps 113 before theelectroplating process. The masks are removed after the electroplatingprocess to obtain the structure shown in FIG. 1.

Copper layer 108 serves as a seed layer for the electroplating of copperlayer 110. Copper layer 108 may be formed to a thickness of about 1600Angstroms by sputtering. Titanium-tungsten layer 106 and aluminum layer104 may also be formed by sputtering. In one embodiment,titanium-tungsten layer 106 and aluminum layer 104 are each formed to athickness of about 1000 Angstroms. Aluminum layer 104 may comprisealuminum with 1% silicon alloy. Silicon dioxide layer 102 serves as adielectric layer providing electrical isolation between the overlyingCu/TiW/Al metal stack and silicon substrate 100. Vias are formed throughsilicon dioxide layer 102 in sections where the Cu/TiW/Al metal stackmakes contact with the doped regions in silicon substrate 100. In oneembodiment, silicon dioxide layer 102 is formed to a thickness of about950 Angstroms.

There may be steps in the fabrication of a solar cell where an etch isperformed through a stack of materials comprising copper,titanium-tungsten, and aluminum. To prevent damaging the solar cell,each layer in the material stack may need to be etched without attacking(i.e., excessively etching) other layers of the solar cell. In theexample of FIG. 1, copper layer 108, titanium-tungsten layer 106, andaluminum layer 104 need to be etched without attacking tin layer 112.Excessive etching of tin layer 112 may compromise its ability to protectthe underlying metal stack.

One way of etching through copper layer 108, titanium-tungsten layer106, and aluminum layer 104 is to use a so-called “PAWN” (phosphoric,acetic, water, nitric) solution to etch copper layer 108 and aluminumlayer 104. For example, the sample of FIG. 1 may be wet etched in a PAWNbath to etch copper layer 108, in a hydrogen peroxide bath to etchtitanium-tungsten layer 106, and in a PAWN bath to etch aluminum layer104. PAWN is commonly used to etch aluminum in the electronics industry,which does not require selectivity to copper, tin, and other materialsemployed in solar cells. The inventors found that PAWN has a tendency toattack tin layer 112 during the etching of copper layer 108 and aluminumlayer 104. The present disclosure provides improved techniques foretching a copper layer, a titanium-tungsten layer, and/or aluminum layerwithout substantially etching a tin layer in a solar cell or similardevice. The techniques may be employed to etch several layers of amaterial stack or a single layer.

Continuing in FIG. 2, copper layer 108 is etched selective to tin layer112 and titanium-tungsten layer 106. Copper layer 108 may be wet etchedin a solution comprising sulfuric acid and hydrogen peroxide. Forexample, copper layer 108 may be wet etched using a Co-Bra Etch® etchantmodified to comprise about 1% by volume of sulfuric acid, about 4% byvolume of phosphoric acid, and about 2% by volume of stabilized hydrogenperoxide. A Perma-Etch® etchant may also be employed to etch copperlayer 108. Co-Bra Etch® and Perma-Etch® etchants are both commerciallyavailable from Electrochemicals, Inc. of Maple Plain, Minn. In oneexperiment, a sample similar to that shown in FIG. 1 was dipped in abath of the aforementioned modified Co-Bra Etch® etchant for about 1minute at 40° C. About 1600 Angstroms of copper layer 108 were removedwithout excessively etching tin layer 112 or titanium-tungsten layer106.

In FIG. 3, titanium-tungsten layer 106 is etched selective to tin layer112, copper layers 110 and 108, and aluminum layer 104.Titanium-tungsten layer 106 may be wet etched in a bath of hydrogenperoxide, for example. The hydrogen peroxide may be 30% by weight(balance is water).

In FIG. 4, aluminum layer 104 is etched selective to tin layer 112,copper layers 110 and 108, titanium-tungsten layer 106, and silicondioxide layer 102. Aluminum layer 104 may be wet etched in a bath ofpotassium hydroxide. In one embodiment, aluminum layer 104 is wet etchedusing an etchant comprising about 1% by volume of potassium hydroxide inwater. Other concentrations of potassium hydroxide may also be employeddepending on the application. In one experiment, a sample similar tothat shown in FIG. 3 was dipped in a bath comprising about 1% by volumeof potassium hydroxide in water for about 1.5 minutes at 40° C. About1000 Angstroms of aluminum layer 104 were removed without excessivelyetching tin layer 112, copper layers 110 and 108, titanium-tungstenlayer 106, and silicon dioxide layer 102.

The teachings of the present disclosure may be generally employed toetch one or more layers of materials in a solar cell being fabricated.For example, the etching techniques disclosed herein may be employed inthe fabrication of solar cells disclosed in the followingcommonly-assigned disclosures, which are incorporated herein byreference in their entirety: U.S. application Ser. No. 10/412,638,entitled “Improved Solar Cell and Method of Manufacture,” filed on Apr.10, 2003 by William P. Mulligan, Michael J. Cudzinovic, Thomas Pass,David Smith, Neil Kaminar, Keith McIntosh, and Richard M. Swanson; andU.S. application Ser. No. 10/412,711, entitled “Metal Contact StructureFor Solar Cell And Method Of Manufacture,” filed on Apr. 10, 2003 byWilliam P. Mulligan, Michael J. Cudzinovic, Thomas Pass, David Smith,and Richard M. Swanson. It is to be noted, however, that theaforementioned disclosures are referenced herein only as examples.

The etch chemistries provided herein not only allow selectivity tomaterials found in solar cells, but also have relatively high etchcapacity, are cost-effective, and are easily replenished and controlled.Embodiments of the present invention may thus be advantageously employedto etch a single layer of material or a stack of materials in solar cellfabrication processes in general.

Referring now to FIG. 5, there is shown a flow diagram of a method 500of etching one or more materials in a solar cell in accordance with anembodiment of the present invention. Among its other uses, method 500may be employed to etch through a material stack comprising copper,titanium-tungsten, and aluminum in a solar cell. Method 500 will bedescribed using FIGS. 1-4 as an example.

In step 502 and with reference to FIG. 1, a plating mask, if any, thatmay have been employed in the electroplating of copper layer 110 isremoved. Step 502 may be performed by placing the sample of FIG. 1 inabout 2% to 3% potassium hydroxide bath at 40° C. for about 5 minutes.

In step 504, the sample of FIG. 1 is rinsed. For example, the sample ofFIG. 1 may be placed in a bath of deionized water, spray-rinsed withdeionized water, and then dumped-rinsed in deionized water.

In step 506, copper layer 108 is etched as shown in FIG. 2. Step 506 maybe performed by placing the sample of FIG. 1 in a bath of a Co-Bra Etch®etchant modified to comprise about 1% by volume of sulfuric acid, about4% by volume of phosphoric acid, and about 2% by volume of stabilizedhydrogen peroxide at 43° C. with robotic agitation for about 2 minutesand 10 seconds .

In step 508, the sample of FIG. 2 is rinsed. Step 508 may be performedby dump-rinsing the sample of FIG. 2 in deionized water.

In step 510, titanium-tungsten layer 106 is etched as shown in FIG. 3.Step 510 may be performed by placing the sample of FIG. 3 in a bath ofabout 30% by weight hydrogen peroxide at 40° C. with robotic agitationfor about 2 minutes and 15 seconds.

In step 512, the sample of FIG. 3 is rinsed. Step 512 may be performedby dump-rinsing the sample of FIG. 3 in deionized water.

In step 514, aluminum layer 104 is etched as shown in FIG. 4. Step 514may be performed by placing the sample of FIG. 3 in a bath of about 1%by volume potassium hydroxide at 40° C. for about 2 minutes and 15seconds.

In step 516, the sample of FIG. 4 is rinsed. Step 516 may be performedby dump-rinsing the sample of FIG. 4 in deionized water.

In step 518, the sample of FIG. 4 is dried. Step 518 may be performed byspin-rinse drying. Air-knife and meniscus drying techniques may also beemployed to dry the sample of FIG. 4.

While specific embodiments of the present invention have been provided,it is to be understood that these embodiments are for illustrationpurposes and not limiting. For example, the above described etchants mayalso be applied using in-line drag-through and in-line spray systems.Many additional embodiments will be apparent to persons of ordinaryskill in the art reading this disclosure.

1-23. (canceled)
 24. An etchant for removing portions of a copper layerin a solar cell without substantially etching a solderable tin layer,the etchant comprising hydrogen peroxide and sulfuric acid.
 25. Theetchant of claim 24 wherein the etchant comprises about 1% by volume ofsulfuric acid, about 4% by volume of phosphoric acid, and about 2% byvolume of stabilized hydrogen peroxide.
 26. An etchant for removingportions of an aluminum layer in a solar cell without excessivelyetching a tin layer, the etchant comprising potassium hydroxide.
 27. Theetchant of claim 26 wherein the etchant comprises about 1% by volume ofpotassium hydroxide in water. 28-32. (canceled)